Thermoplastic elastomers display the properties of rubber and can be processed as thermoplastics. In other words, they can be processed into articles of manufacture utilizing conventional thermoplastic processing techniques, such as injection molding or good extrusion. Thermoplastic elastomers generally exhibit flexibility, elastic recovery and resilience.
It is advantageous to utilize thermoplastic elastomers instead of thermosetting rubbers where possible to reduce processing costs. This is because the process cycle time with thermoplastic elastomers is less than the processing cycle time of conventional rubbers. Additionally, the scrap which is generated in the processing of thermoplastic elastomers can be readily recycled. This is in contrast to operations which utilize thermosetting rubbers wherein scrap which is generated after curing cannot be reprocessed. For these reasons, thermoplastic elastomers generally allow for much more efficient plant and equipment utilization than do thermosetting rubbers.
Various blends of rubbers with thermoplastic materials have been prepared which exhibit the properties of thermoplastic elastomers. Such blends are described in U.S. Pat. No. 4,104,210, U.S. Pat. No. 4,130,535, and U.S. Pat. No. 4,355,139. Such blends are also described in detail in Coran and Patel, Rubber-Thermoplastic Compositions, Part I EPDM-Polypropylene Thermoplastic Vulcanizates, 53 Rubber Chemistry and Technology, 141-150 and Coran, Patel, and Williams, Rubber-Thermoplastic Compositions, Part V Selecting Polymers for Thermoplastic Vulcanizates, 55 Rubber Chemistry and Technology, 116-136. One of the most useful types of blends described in these references is blends of EPDM rubber with polypropylene. Such blends have superior strength, high-temperature mechanical properties and good compression set. However, such blends exhibit only fair solvent resistance. Coran describes the dynamic vulcanization of such blends and is herein incorporated by reference in its entirety.
Nitrile rubbers are known to exhibit excellent resistance to organic solvents, such as oils. However, the simple blending of EPDM rubber with nitrile rubber leads to poor physical properties because the two polymers are incompatible. Thus, it is not possible to improve the solvent resistance of EPDM/polypropylene blends by simply incorporating a nitrile rubber therein without destroying the desirable physical properties of such blends.